The invention relates to gels selected from among lyogel- or aerogel-containing primary particles which are made up of oxidic units and [RxSiO(4-x)/2] units, wherein the primary particles have a change in the concentration of [RxSiO(4-x)/2] units from the inside to the outside, where the indices x can be identical or different and are in each case 1 or 2 and the radicals R can be identical or different and are each hydrogen or an organic, substituted or unsubstituted radical, and the oxidic units contain [SiO4/2] units, and also a process for the production thereof.
The invention further relates to the use of the gels of the invention in cosmetic or medical applications, as catalyst or catalyst support, for chromatographic applications, or, when the gels are aerogels, for thermal and/or acoustic insulation.
Thermal insulation to save energy has attained a prominent position in the context of concern for sustainable development and the increasing cost of energy and also ever scarcer fossil raw materials. These requirements for optimization of thermal insulation protection apply equally to buildings, i.e. to new builds or existing buildings, and also to thermal insulation in the logistic or stationary sector.
In the context of durable insulation which has a low thermal conductivity and also a low combustibility, the focus is increasingly on inorganic porous materials.
Aerogels having high porosities (>60%) and a low density (<0.6 g/ml) have a low thermal conductivity and are therefore widely used as thermal insulators (M. A. Aegerter et al. (Eds.), Aerogels Handbook Series: Advances in Sol-Gel Derived Materials and Technologies, 1st ed. 2011, Springer publishers, New York Dordrecht Heidelberg London). Gels, in particular SiO2 gels, are built up of networks which are composed of primary particles which, after linkage and sintering of the contact areas in a sol-gel process, form stable networks filled with liquid, known as lyogels. These lyogels can be converted into aerogels by removal of the solvent. The pores of the aerogel are accordingly filled with air. While the pores are filled with solvent in the case of a lyogel, a hydrogel is a specific case of a lyogel, in which the pore liquid comprises at least 50% of water.
It is desirable to achieve a very high hydrophobicity of SiO2 aerogels in order to reduce the water absorption and as a result the reduction in the thermal insulation effect. Permanent hydrophobicity is achieved by treatment of the surface of gel networks with hydrophobic groups, preferably by modification.
In addition, it is desirable to produce aerogels having a very low combustibility and therefore a very low carbon content (C content).
For further applications in thermal insulation, a very high stability in respect of mechanical influences and flexibility of the thermal insulation materials to be used are also desirable.
Inorganic aerogels, especially those based on SiO2, have been known since 1931 (see, for example, Aegerter et al., Aerogels Handbook 2011, see above). These are produced by formation of a network of SiO2 primary particles by means of a sol-gel process.
EP 0 948 395 B1 describes the production of hydrophobic aerogels based on SiO2. Here, network formation occurs in aqueous solutions to give hydrogels which are subsequently modified by silylation of the accessible Si—OH groups on the surface and are then dried subcritically to give aerogels. The post-silylation leads to a degree of coverage with trimethylsilyl groups (TMS, (CH3)3Si—) of at least 2.6 nm−2, and the C content is accordingly in no case below 6.8% by weight (see table 1).
In addition, these networks have the disadvantages which are generally known in the prior art for aerogels, namely that they are crumbly and brittle (see, for example, introduction of A. V. Rao et al., J. Colloid Interface Sci. 300, 2006, p. 279-285; or Aegerter et al. Aerogels Handbook 2011, see above) since they consist of rigid SiO2 frameworks.
Covering the surface of fully formed SiO2 gel networks (as hydrogel or lyogel) with other silanes, by which means the rigid SiO2 framework is covered with a layer of [RxSiO(4-x)/2] (where x=1 or 2), is likewise known from the prior art.
However, this construction has the disadvantage that post-modification of the gel network with alkoxysilanes to reinforce the network is associated with multistage and thus complicated solvent replacement steps (A. V. Rao et al., Appl. Surf. Sci. 253, 2007, p. 6032-6040). Since the primary particles are directly joined, i.e. the contact points between the primary particles are in this case, too, formed precisely as described in EP 0 948 395 by rigid SiO2 bridges due to the production method, this process likewise gives brittle products (see FIG. 1). Since the interparticulate linkages are formed before modification, these are made up of [SiO4/2] units (depicted with a light color in FIG. 1).
An alternative approach, namely the direct buildup of aerogels composed of [CH3SiO3/2] units without subsequent silylation, has been described in the publications by A. V. Rao et al. (2006, see above) and K. Kanamori et al. (Adv. Mater. 19, 2007, p. 1589-1593). Highly flexible aerogels were obtained as products. However, these have a high C content which was calculated as 17.9% by weight despite the omission of subsequent silylation due to the high carbon content of the starting material used. The combustibility of these gels is thus increased compared to the products of EP 0 948 395 (see FIG. 2). The entire gel network and thus also the interparticulate linkages consist of [RxSiO(4-x)/2] units (depicted in black in FIG. 2). Depending on the starting materials used, the gel therefore has a very high C content.